1. Field of the Invention
The invention relates to methods for measuring the optical path difference of an imbalanced interferometer using a wavelength modulated laser. The invention also relates to a system for carrying out the method.
2. Description of Prior Art
In my co-pending U.S. application Ser. No. 499,798, filed Mar. 27, 1990, I describe a method and system for measuring the optical path difference of an imbalanced interferometer using a wavelength modulated laser wherein the frequency of the optical fringes generated by modulating the wavelength of the laser is measured to determine the optical path difference. To carry out the method in the application, it is necessary to have a laser which can be tuned through a wide range of wavelengths without mode hopping. Such lasers are expensive which presents some restrictions in the commercial implementation of the invention of the application.
Other interferometric measurement systems are also known in the art as illustrated in, for example. U.S. Pat. No. 4,874,244, Kersey, Oct. 17, 1989, U.S. Pat. No. 4,552,457, Giallorenzi et al, Nov. 12, 1985, and U.S. Pat. No. 4,594,003, Sommargren, June 10, 1986.
The '244 patent uses two lasers 4 and 5 (see FIG. 1) operating at different wavelengths .lambda..sub.1 and .lambda..sub.2. The signals from the two lasers are combined in couplers 6 and 7, and the combined signal is introduced to an interferometric optical fiber system. An output of the interferometric optical fiber system is detected and divided into portions attributable to .lambda..sub.1 and .lambda..sub.2, and the phase lag between the output signals is measured. Lasers 4 and 5 may be modulated with signals f.sub.1 and f.sub.2.
As seen in the system diagram illustrated in FIG. 4 of the '457 patent, this system also uses two lasers with different wavelengths. The interferometer, as seen in FIG. 1, comprises fiber optic reference arm and difference length fiber optic sensing arm 14 and 12 respectively. The output of the interferometer is split into third and fourth beams to determine which lasers should be energized.
U.S. Pat. No. 4,594,003 teaches only a single laser 11. However, in accordance with the teachings in this patent, this laser can have its wavelength varied by current source 58.